1. Field of the Invention
The present invention relates to a highly efficient reflecting plate used for a light-emitting device, and also relates to an organic electroluminescence element excellent in light-extraction efficiency employing the same.
2. Background Art
In an organic electroluminescence element (hereinafter, often referred to as “organic EL element”), the refractive index of the light-emitting part is generally approx. 2.0, which is higher than that of air 1.0. Accordingly, because of total reflection at the interface between the element and air, approx. 80% of the emitted light is confined and travels in the element, and hence cannot be extracted out (see, C. F. Madigan, M. H. Lu, J. C. Strum, Applied Physics Letters, Vol. 76, 1650(2000)). If a light-extraction layer is provided so as to extract out the light traveling in the element, the organic EL element can be improved in efficiency. The problem of the confined light is liable to occur not only in an organic EL element but also in other devices, such as liquid crystal displays and lighting devices, which comprise light-emitting parts having higher refractive indexes than air.
It is proposed to form a two-dimensional diffraction structure as the light-extraction layer in the element (see, JP-A H11-283751 (KOKAI), for example). That structure diffracts the light traveling in the element, and thereby the confined light, which hitherto has not been extracted out, can be partly extracted out of the element. As a result, the light-extraction efficiency is improved as compared with the element not provided with the diffraction structure. However, the two-dimensional diffraction structure has too small diffraction efficiency to sufficiently improve the light-extraction efficiency. Further, the diffraction structure is a submicron-order periodical structure and is generally formed in the organic EL element by the photo-lithographic process, which is used for micro-fabrication of semiconductors. Accordingly, an expensive apparatus and complicated procedures are necessary and hence it costs a lot to produce the diffraction structure. In addition, since the periodical structure causes uneven brightness on the light-emitting surface, it is necessary to take some measures when applied to displays or other devices required to have uniformity of brightness on the light-emitting surface.
It is also proposed to provide a light-scattering layer as the light-extraction layer between the substrate and the light-emitting layer in the organic EL element (see, JP-A 2006-107744 (KOKAI), for example). The light-scattering layer comprises a transparent resin and fine particles dispersed therein, and the fine particles have a refractive index different from that of the resin. In the element, light emitted from the light-emitting part is scattered by the light-scattering layer and the scattered light travels in various directions. As a result of the multiple scattering, light incident to the interface to air at an angle within the range of the total reflection angle is extracted out. Since the light-scattering layer changes the traveling direction of light at random, it is preferred that the fine particles have a broad size distribution, be arranged at random and have a large volume fraction in the scattering layer. If the size distribution is narrow or if the volume fraction is small, the light-scattering layer cannot scatter the light sufficiently. On the other hand, if the size distribution is broad, it is difficult to ideally arrange the particles in the resin. Further, if the particles having a broad size distribution are incorporated in a large volume fraction, the obtained light-scattering layer is remarkably poor in flatness and accordingly the light-emitting part in the form of a thin layer is also poor in flatness to lower reliability of the EL element considerably. Thus, this is a dilemma. In a conventional EL element with the light-scattering layer, therefore, the volume fraction is controlled at approx. 20% and hence the effect of scattering is so small that it cannot be expected to improve the light-extraction efficiency remarkably.
As described above, it is difficult to remarkably improve the light-extraction efficiency of an organic EL element by conventional technology, and accordingly it is desired to improve the efficiency.